Balloons mounted on the distal ends of catheters are widely used in medical treatment. The balloon may be used to widen a vessel into which the catheter is inserted or to force open a blocked vessel. The requirements for strength and size of the balloons vary widely depending on the balloon's intended use and the vessel size into which the catheter is inserted.
Perhaps the most demanding applications for such balloons are in balloon angioplasty in which catheters are inserted for long distances into extremely small vessels and used to open stenoses of blood vessels by balloon inflation. These applications require extremely thin walled, high strength, relatively inelastic balloons of predictable inflation properties. Thin walls are necessary because the balloon's wall and waist thicknesses limit the minimum diameter of the distal end of the catheter and therefore determine the limits on vessel size treatable by the method and the ease of passage of the catheter through the vascular system. High strength is necessary because when the balloon is used to push open a stenosis, the thin wall must not burst under the high internal pressures necessary to accomplish this task. The balloon must have some elasticity so that the inflated diameter can be controlled, enabling the surgeon to vary the balloon's diameter as required to treat individual lesions, but that elasticity must be relatively low so that the diameter is easily controllable. Small variations in pressure must not cause wide variation in diameter. Such angioplasty balloons have nominal diameters in the range of from about 1.25–4.5 mm.
Outside the field of angioplasty, however, relatively high compliant, high strength materials are desirable for some balloons used on esophageal, pyloric, colonic and anastomotic catheters and scopes.
Major advances in the ability to access remote areas within the gastrointestinal tract have allowed endoscopists to reach obstructive lesions previously accessible only via open surgical techniques. There are three primary approaches available to the clinician for endoscopic treatment of gastrointestinal strictures: 1) Mercury bougie dilatation; 2) Over-the-wire passage of tapered dilators; and 3) Balloon dilation.
Of the three, balloon dilatation is the most recently developed modality. Clinical research studies have been conducted to compare the efficacy of the technique to earlier approaches. For example, in one study evaluating Savary-Guillard® Dilators versus balloon dilators for dilatation of benign esophageal strictures, both methods achieved effective dilatation. However, during the 24 month follow-up, 88% of patients treated with Savary dilators required redilations vs 54% of patients in the balloon group. As a result, the researchers concluded that, over the long term, the balloon may provide superior efficacy. Additional studies have clearly documented the convenience, effectiveness and safety of balloon dilatation of strictures.
An important advantage of balloon dilatation over the alternative techniques is that it enables the clinician to dilate remote strictures throughout the GI tract.
An example is the treatment of esophagal achalasia. The esophagus, a hollow, muscular organ that originates at the pharynx and terminates at the stomach, functions to transport food and fluids from the pharynx to the stomach via a complex, neuromuscular response to the act of swallowing. Specifically, the passage of food or fluid from the pharynx into the esophagus stimulates the peristaltic contractions designed to propel the contents forward through the esophagus. Concurrently, the lower esophageal sphincter (LES) at the gastroesophagal junction relaxes allowing the passage of esophagal contents into the stomach. Reflux of stomach contents back into the esophagus is prevented by closure of the LES. Achalasia is a disorder of unknown etiology that disrupts the normal esophageal function (3,4). In this disorder, two deficits are present. First, the normal esophagal peristaltic wave is absent. Second, the lower esophageal sphincter does not relax. The result is esophageal dilatation and severe, progressive dysphagia. Treatment for achalasia is aimed at reduction of LES pressure. This is accomplished nonsurgically via forceful balloon dilatation of the sphincter.
Biliary dilatation may also be performed by such balloon catheter dilatation. Biliary strictures may result from variety of processes including postoperative scarring, inflammation, or malignancy. Endoscopic balloon dilatation of these lesions has been shown to be an effective treatment approach.
There therefore is a need for effective devices which permit endoscopic dilatation of lesions throughout the alimentary tract. It is important that the catheters offer first-use effectiveness in an advanced design to permit rapid inflation, deflation and easy scope passage. The balloons for such devices desirably would have a long dilation length, high operating pressure, typically greater than 50 psi (3 atm, 344.7 kPa) and desirably up to 146 psi, (10 atm, 1013 kPa), low withdrawal force and high compliance. For instance, a compliance change is desirable which would allow a balloon having a 1.25–3.0 mm nominal diameter at 3 atm to grow in a generally linear manner at least 0.25 mm, preferably about 0.5 mm, or more as pressure is increased from 3 to 12 atm. For balloons about 3.25–6.0 mm nominal diameter, a growth of at least 1.0 mm over the same range would be desirable. For balloons in the range of about 6–12 mm nominal diameter, a growth of at least 2.0 mm over a 3–10 atm pressure range would be desirable. For even larger diameter balloons, for instance balloons having 3 atm diameters of 12–30 mm, a compliance curve which provides growth of about 3 mm or more, preferably about 4.0 mm or more, over the range of 3 to 9 atm is desirable.
In U.S. Pat. No. 5,348,538, incorporated herein by reference, there is described a single layer angioplasty balloon made of a material such as PET which follows a stepped compliance curve. The stepped compliance curves of these balloons have a low pressure segment following a first generally linear profile, a transition region, typically in the 8–14 atm range, during which the balloon rapidly expands yielding inelastically, and a higher pressure region in which the balloon expands along a generally linear, low compliance curve. The stepped compliance curve allows a physician to dilate different sized lesions without using multiple balloon catheters.
A polyethylene ionomer balloon with a stepped compliance curve is disclosed in EP 540 858.
In copending U.S. application Ser. No. 08/392,837, filed 2 Mar. 1995, entitled Block Copolymer Elastomer Catheter Balloons, incorporated herein by reference, which corresponds to WO 95/23619, there are described balloons, useful on angioplasty catheters and similar medical devices, which are made from certain block copolymer materials which provide an unusual combination of compliance, softness and strength properties.
Block copolymer balloons for balloon catheters, prepared using a particular heat set technique to stabilize the balloon dimensions, are also described in U.S. Pat. No. 5,500,180.